The present invention relates to a highly compact laser scanning microscope with integrated short-pulse laser.
Laser scanning microscopes using short-pulse lasers (e.g., picosecond or femtosecond lasers) have long been known, primarily in time-solved and multiphoton microscopy.
WO 91/07651 discloses a two-photon laser scanning microscope with excitation through laser pulses in the subpicosecond range at excitation wavelengths in the red or infrared region.
EP 666473A1 WO 95/30166, DE 4414940 A1 describe excitations in the picosecond range, and beyond, with pulsed or continuous radiation.
A process for optical excitation of a specimen by means of two-photon excitation is described in DE C2 4331570.
DE 29609850 by the present Applicant describes coupling of the radiation of short-pulse lasers into a microscope beam path via light-conducting fibers.
The short-pulse lasers that are used (wavelength-tunable lasers as well as fixed wavelength lasers) are, among other things, very bulky and, because of their great technical complexity, very expensive and can only be handled with difficulty by the user. In general, lasers are optically coupled directly into the scan module of the laser scanning microscope by means of beam guiding systems (e.g., mirrors or glass prisms). This generally requires long optical beam paths which renders the system sensitive to adjustment and results in very large systems because of the extent of the optical beam path. The frequently required adjustment of the laser (particularly also when the laser wavelength is changed or tuned) results, among other things, in spatial drifting of the laser beam coupled out of the laser. Consequently, the laser beam is no longer optimally adjusted within the laser scanning microscope. The latter problem can be avoided by means of a fiber coupling (e.g., using single-mode, polarization-conserving fibers) of the short-pulse laser, so that the laser and laser scanning microscope are adjusted in isolation from one another. This kind of fiber coupling of a short-pulse laser in a laser scanning microscope is basically possible, but is very problematic for the short pulses due to the optical scattering of the glass fibers and nonlinear optical effects such as self-phase modulation, Brillouin scattering, Raman scattering, etc. which can occur at the high laser intensities in the glass fiber core.
It is object of the present invention to provide a highly compact laser scanning microscope with a short-pulse laser integrated in the scan module. Optical direct coupling or a fiber coupling of the short-pulse laser with the laser scanning microscope can be advantageously circumvented with this arrangement.
Short-pulse lasers which can be engineered in a highly compact constructional form of this kind are, for example, diode laser pumped ion-doped fiber lasers, e.g., diode pumped Er3 + doped fiber lasers with a laser emission from the resonator cavity at a wavelength of about 1550 nm. This laser radiation can be frequency-doubled in a highly efficient manner (resonant, nonresonant, quasi-phase matching) to a wavelength of about 790 nm outside of the laser resonator cavity by means of nonlinear optical crystals due to the high laser intensities. A portion of the radiation is also converted in the crystal to the third harmonic at approximately 515 nm and to the fourth harmonic at approximately 387 nm. All other conceivable nonlinear conversion processes also occur in the crystal with a determined conversion efficiency. Accordingly, for example, a highly compact short-pulse laser with several different fixed output wavelengths simultaneously is made available for diverse microscope applications.
The laser can be furnished, for instance, in a compact housing which is fastened in the chassis of the scan module and the laser beam can be emitted from an opening in the compact housing, possibly so as to be adjustable (FIG. 1). The laser beam can pass through an optical system arranged outside of the compact housing which transforms it to a suitable beam diameter and beam divergence. This optical system can be constructed variably such that it is suitable for adapting the beam to the chromatic characteristics of the microscope optical system (e.g., a variable beam expander); it can also be constructed in such a way that it is suitable for adapting the beam diameter to the diameter of the objective pupil for optimizing the ratio of transmission to spatial resolution (variable zoom).
A laser shutter (beam interrupter) for ensuring laser safety in the device is to be integrated in the laser beam path in the scan module of the laser scanning microscope. This can be constructed, for example, as a mechanical beam interrupter.
The laser radiation can be guided through a wavelength-selective optical element for selecting the laser wavelength required by the application. This element can be constructed as a dielectric filter, an acousto-optical, electro-optical, refractive, or dispersive element or as a combination of these.
The laser radiation can be guided through an intensity-damping element for adjusting the laser output required by the application. This element can be constructed as a neutral filter, as an acousto-optical, electro-optical, refractive, or dispersive element or as a combination thereof. In the case of an acousto-optical element, this can also be advantageously used as a pulse picker for varying the pulse repetition rate of the laser (temporal isolation of individual laser pulses from the continuous train of laser pulses).
A prechirp unit, e.g., comprising a sequence of gratings or prisms or a combination thereof, can be inserted in the laser beam path (illumination beam path) in order to provide negative dispersion for compensating for the positive dispersion of the optical system including the scan module, microscope and specimen. Accordingly, it is possible in an advantageous manner to provide laser pulses at the location of the specimen which are transform-limited as far as possible.
The laser radiation then impinges on a main beam splitter (e.g., a dielectric color splitter) which steers the laser radiation in the direction of the specimen being analyzed. This main beam splitter which can be constructed as one of many color splitters in a main beam splitter turret can also be constructed, at the same time, as the wavelength-selective optical element for selecting the laser wavelength required by the application. In the case of nonoptical detection techniques (e.g., OBIC or LIVA), the main beam splitter can also be constructed as a fully reflecting mirror.
Galvanometer scanners can be used as laser beam scanners for deflecting the beam in the x- and y-direction. In the case of multiphoton microscopy, the fluorescence is excited in the specimen by one or more of the available laser wavelengths. The detection of the fluorescence radiation is then carried out in part in a multichannel arrangement according to FIG. 2 in which the use of confocal pinholes for achieving three-dimensional resolution is not necessary. In one variant, however, confocal pinholes can be used in order to further increase the depth resolution beyond that of simple multiphoton microscopy (FIG. 3).
Detection of an optical or nonoptical signal in descanned or non-descanned detector configuration.
Depending on the pulse-width repetition rate of the pulsed laser radiation, a lock-in amplifier or a boxcar amplifier can be used in synchronization with the pulse repetition rate of the laser for phase-sensitive detection of the detection signal. This results in a considerably improved signal-to-noise ratio by reduction of the bandwidth in the detection system in which noise is detected.
The use of wavelengths around 1550 nm is pertinent, for example, in microscopic inspection of semiconductors, especially structured silicon wafers. Wavelengths around 1550 nm are also still favorably transmitted through highly doped silicon. Only in the immediate area of the objective focus, and thus with depth discrimination, is there an excitation of nonlinear optical processes such as multiphoton excitation (e.g., also OBIC or LIVA as nonoptical detection techniques) or higher-harmonic generation. These nonlinear processes can accordingly also be used as a microscopic contrasting method in connection with thick silicon substrates for three-dimensionally resolved microscopy, e.g., in the field of nondestructive testing of wafers.
The wavelength around 790 nm is suitable, for example, especially for universal excitation of two-photon processes in dyes conventionally used for fluorescence tagging of biological specimens.
The green wavelength and ultraviolet wavelength at 517 nm and 387 nm, respectively, can be used, for example, for analyzing specimens in reflection contrast, fluorescence contrast, for time-resolved microscopy. The green radiation at 517 nm can also be used as a visible pilot beam for adjustment when assembling the optical system.
In particular, the described system is suitable for use in physiological inquiries, e.g., for releasing caged compounds. In this case, radiation at 790 nm can be utilized by means of three-photon processes for uncaging even in deep layers of thick specimens, while the observation of the liberated ions is then carried out by two-photon excitation of the fluorophores used for tagging.
The invention is characterized by the following particularly advantageous central features:
integration of a short-pulse laser in the scan module of a laser scanning microscope for providing a highly compact microscope system;
integration of a (frequency-tunable or wavelength-tunable) short-pulse laser with an optical arrangement following the laser resonator cavity and comprising one or more nonlinear optical crystals for frequency conversion of the laser radiation by means of frequency doubling, frequency tripling, sum frequency generation, difference frequency generation, another optically parametric process, or any desired combination thereof, for providing a plurality of wavelengths for diverse microscope applications;
its use in time-resolved laser scanning microscopy;
its use in confocal or nonconfocal laser scanning microscopy using an optical detection signal;
its use in laser scanning microscopy using a nonoptical detection signal;
its use in laser scanning two-photon microscopy;
its use in material analysis, especially frequency doubling (SHG) at surfaces by means of a laser scanning microscope;
its use in material analysis, especially for two-dimensional or three-dimensional OBIC;
combined use of two or more of the techniques described above.